High dynamic range imaging pixels with multiple photodiodes

ABSTRACT

A high dynamic range imaging pixel may include first and second photodiodes that generate charge in response to incident light. The second photodiode may have a higher sensitivity than the first photodiode. When generated charge in the first photodiode exceeds a given charge level, the charge may overflow through a transistor to a capacitor. The overflow path from the first photodiode to the capacitor may optionally pass through the floating diffusion region. A transistor may be coupled between the first and second photodiodes. A gain select transistor may be coupled between the floating diffusion region and the capacitor. After sampling the overflow charge, the charge from both the first and second photodiodes may be sampled. In one arrangement, overflow charge may be transferred to a capacitor in a subsequent row.

BACKGROUND

This relates generally to imaging devices, and more particularly, toimaging devices having high dynamic range imaging pixels.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. In a typicalarrangement, an image sensor includes an array of image pixels arrangedin pixel rows and pixel columns. Circuitry may be coupled to each pixelcolumn for reading out image signals from the image pixels. Typicalimage pixels contain a photodiode for generating charge in response toincident light. Image pixels may also include a charge storage regionfor storing charge that is generated in the photodiode. Image sensorscan operate using a global shutter or a rolling shutter scheme.

Some conventional image sensors may be able to operate in a high dynamicrange (HDR) mode. HDR operation may be accomplished in image sensors byassigning alternate rows of pixels different integration times. However,conventional HDR image sensors may sometimes experience lower thandesired resolution, lower than desired sensitivity, higher than desirednoise levels, and lower than desired quantum efficiency.

It would therefore be desirable to be able to provide improved highdynamic range operation in image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having an imagesensor in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals in an image sensor inaccordance with an embodiment.

FIG. 3 is a circuit diagram of an illustrative imaging pixel thatincludes an overflow capacitor and connected photodiodes havingdifferent sensitivities in accordance with an embodiment.

FIG. 4 is a timing diagram showing an illustrative method of operationfor the imaging pixel of FIG. 3 in accordance with an embodiment.

FIG. 5 is a circuit diagram of an illustrative imaging pixel thatincludes an overflow capacitor and a low-sensitivity photodiode that isadjacent to a floating diffusion region in accordance with anembodiment.

FIG. 6 is a timing diagram showing an illustrative method of operationforthe imaging pixel of FIG. 5 in accordance with an embodiment.

FIG. 7 is a circuit diagram of an illustrative imaging pixel thatincludes an overflow capacitor that receives overflow charge from animaging pixel in a previous row in accordance with an embodiment.

FIG. 8 is a timing diagram showing an illustrative method of operationfor the imaging pixel of FIG. 7 in accordance with an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors. It will berecognized by one skilled in the art that the present exemplaryembodiments may be practiced without some or all of these specificdetails. In other instances, well-known operations have not beendescribed in detail in order not to unnecessarily obscure the presentembodiments.

Electronic devices such as digital cameras, computers, cellulartelephones, and other electronic devices may include image sensors thatgather incoming light to capture an image. The image sensors may includearrays of pixels. The pixels in the image sensors may includephotosensitive elements such as photodiodes that convert the incominglight into image signals. Image sensors may have any number of pixels(e.g., hundreds or thousands or more). A typical image sensor may, forexample, have hundreds of thousands or millions of pixels (e.g.,megapixels). Image sensors may include control circuitry such ascircuitry for operating the pixels and readout circuitry for reading outimage signals corresponding to the electric charge generated by thephotosensitive elements.

FIG. 1 is a diagram of an illustrative imaging and response systemincluding an imaging system that uses an image sensor to capture images.System 100 of FIG. 1 may be an electronic device such as a camera, acellular telephone, a video camera, or other electronic device thatcaptures digital image data, may be a vehicle safety system (e.g., anactive braking system or other vehicle safety system), may be asurveillance system, or may be any other desired type of system.

As shown in FIG. 1, system 100 may include an imaging system such asimaging system 10 and host subsystems such as host subsystem 20. Imagingsystem 10 may include camera module 12. Camera module 12 may include oneor more image sensors 14 and one or more lenses.

Each image sensor in camera module 12 may be identical or there may bedifferent types of image sensors in a given image sensor arrayintegrated circuit. During image capture operations, each lens may focuslight onto an associated image sensor 14. Image sensor 14 may includephotosensitive elements (i.e., pixels) that convert the light intodigital data. Image sensors may have any number of pixels (e.g.,hundreds, thousands, millions, or more). A typical image sensor may, forexample, have millions of pixels (e.g., megapixels). As examples, imagesensor 14 may include bias circuitry (e.g., source follower loadcircuits), sample and hold circuitry, correlated double sampling (CDS)circuitry, amplifier circuitry, analog-to-digital converter circuitry,data output circuitry, memory (e.g., buffer circuitry), addresscircuitry, etc.

Still and video image data from camera sensor 14 may be provided toimage processing and data formatting circuitry 16 via path 28. Path 28may be a connection through a serializer/deserializer (SERDES) which isused for high speed communication and may be especially useful inautomotive systems. Image processing and data formatting circuitry 16may be used to perform image processing functions such as dataformatting, adjusting white balance and exposure, implementing videoimage stabilization, face detection, etc. Image processing and dataformatting circuitry 16 may also be used to compress raw camera imagefiles if desired (e.g., to Joint Photographic Experts Group or JPEGformat). In a typical arrangement, which is sometimes referred to as asystem on chip (SOC) arrangement, camera sensor 14 and image processingand data formatting circuitry 16 are implemented on a commonsemiconductor substrate (e.g., a common silicon image sensor integratedcircuit die). If desired, camera sensor 14 and image processingcircuitry 16 may be formed on separate semiconductor substrates. Forexample, camera sensor 14 and image processing circuitry 16 may beformed on separate substrates that have been stacked.

Imaging system 10 (e.g., image processing and data formatting circuitry16) may convey acquired image data to host subsystem 20 over path 18.Path 18 may also be a connection through SERDES. Host subsystem 20 mayinclude processing software for detecting objects in images, detectingmotion of objects between image frames, determining distances to objectsin images, filtering or otherwise processing images provided by imagingsystem 10.

If desired, system 100 may provide a user with numerous high-levelfunctions. In a computer or advanced cellular telephone, for example, auser may be provided with the ability to run user applications. Toimplement these functions, host subsystem 20 of system 100 may haveinput-output devices 22 such as keypads, input-output ports, joysticks,and displays and storage and processing circuitry 24. Storage andprocessing circuitry 24 may include volatile and nonvolatile memory(e.g., random-access memory, flash memory, hard drives, solid-statedrives, etc.). Storage and processing circuitry 24 may also includemicroprocessors, microcontrollers, digital signal processors,application specific integrated circuits, etc.

An example of an arrangement for camera module 12 of FIG. 1 is shown inFIG. 2. As shown in FIG. 2, camera module 12 includes image sensor 14and control and processing circuitry 44. Control and processingcircuitry 44 may correspond to image processing and data formattingcircuitry 16 in FIG. 1. Image sensor 14 may include a pixel array suchas array 32 of pixels 34 (sometimes referred to herein as image sensorpixels, imaging pixels, or image pixels 34) and may also include controlcircuitry 40 and 42. Control and processing circuitry 44 may be coupledto row control circuitry 40 and may be coupled to column control andreadout circuitry 42 via data path 26. Row control circuitry 40 mayreceive row addresses from control and processing circuitry 44 and maysupply corresponding row control signals to image pixels 34 over controlpaths 36 (e.g., dual conversion gain control signals, pixel resetcontrol signals, charge transfer control signals, blooming controlsignals, row select control signals, or any other desired pixel controlsignals). Column control and readout circuitry 42 may be coupled to thecolumns of pixel array 32 via one or more conductive lines such ascolumn lines 38. Column lines 38 may be coupled to each column of imagepixels 34 in image pixel array 32 (e.g., each column of pixels may becoupled to a corresponding column line 38). Column lines 38 may be usedfor reading out image signals from image pixels 34 and for supplyingbias signals (e.g., bias currents or bias voltages) to image pixels 34.During image pixel readout operations, a pixel row in image pixel array32 may be selected using row control circuitry 40 and image dataassociated with image pixels 34 of that pixel row may be read out bycolumn control and readout circuitry 42 on column lines 38.

Column control and readout circuitry 42 may include column circuitrysuch as column amplifiers for amplifying signals read out from array 32,sample and hold circuitry for sampling and storing signals read out fromarray 32, analog-to-digital converter circuits for converting read outanalog signals to corresponding digital signals, and column memory forstoring the read out signals and any other desired data. Column controland readout circuitry 42 may output digital pixel values to control andprocessing circuitry 44 over line 26.

Array 32 may have any number of rows and columns. In general, the sizeof array 32 and the number of rows and columns in array 32 will dependon the particular implementation of image sensor 14. While rows andcolumns are generally described herein as being horizontal and vertical,respectively, rows and columns may refer to any grid-like structure(e.g., features described herein as rows may be arranged vertically andfeatures described herein as columns may be arranged horizontally).

Pixel array 32 may be provided with a color filter array having multiplecolor filter elements which allows a single image sensor to sample lightof different colors. As an example, image sensor pixels such as theimage pixels in array 32 may be provided with a color filter array whichallows a single image sensor to sample red, green, and blue (RGB) lightusing corresponding red, green, and blue image sensor pixels arranged ina Bayer mosaic pattern. The Bayer mosaic pattern consists of a repeatingunit cell of two-by-two image pixels, with two green image pixelsdiagonally opposite one another and adjacent to a red image pixeldiagonally opposite to a blue image pixel. In another suitable example,the green pixels in a Bayer pattern are replaced by broadband imagepixels having broadband color filter elements (e.g., clear color filterelements, yellow color filter elements, etc.). These examples are merelyillustrative and, in general, color filter elements of any desired colorand in any desired pattern may be formed over any desired number ofimage pixels 34.

If desired, array 32 may be part of a stacked-die arrangement in whichpixels 34 of array 32 are split between two or more stacked substrates.In such an arrangement, each of the pixels 34 in the array 32 may besplit between the two dies at any desired node within the pixel. As anexample, a node such as the floating diffusion node may be formed acrosstwo dies. Pixel circuitry that includes the photodiode and the circuitrycoupled between the photodiode and the desired node (such as thefloating diffusion node, in the present example) may be formed on afirst die, and the remaining pixel circuitry may be formed on a seconddie. The desired node may be formed on (i.e., as a part of) a couplingstructure (such as a conductive pad, a micro-pad, a conductiveinterconnect structure, or a conductive via) that connects the two dies.Before the two dies are bonded, the coupling structure may have a firstportion on the first die and may have a second portion on the seconddie. The first die and the second die may be bonded to each other suchthat first portion of the coupling structure and the second portion ofthe coupling structure are bonded together and are electrically coupled.If desired, the first and second portions of the coupling structure maybe compression bonded to each other. However, this is merelyillustrative. If desired, the first and second portions of the couplingstructures formed on the respective first and second dies may be bondedtogether using any metal-to-metal bonding technique, such as solderingor welding.

As mentioned above, the desired node in the pixel circuit that is splitacross the two dies may be a floating diffusion node. Alternatively, thedesired node in the pixel circuit that is split across the two dies maybe the node between a floating diffusion region and the gate of a sourcefollower transistor (i.e., the floating diffusion node may be formed onthe first die on which the photodiode is formed, while the couplingstructure may connect the floating diffusion node to the source followertransistor on the second die), the node between a floating diffusionregion and a source-drain node of a transfer transistor (i.e., thefloating diffusion node may be formed on the second die on which thephotodiode is not located), the node between a source-drain node of asource follower transistor and a row select transistor, or any otherdesired node of the pixel circuit.

In general, array 32, row control circuitry 40, column control andreadout circuitry 42, and control and processing circuitry 44 may besplit between two or more stacked substrates. In one example, array 32may be formed in a first substrate and row control circuitry 40, columncontrol and readout circuitry 42, and control and processing circuitry44 may be formed in a second substrate. In another example, array 32 maybe split between first and second substrates (using one of the pixelsplitting schemes described above) and row control circuitry 40, columncontrol and readout circuitry 42, and control and processing circuitry44 may be formed in a third substrate.

To increase high dynamic range in imaging pixels, an imaging pixel mayinclude first and second photosensitive areas with differentsensitivities. A high sensitivity photodiode may be optimal for sensorperformance in low light conditions whereas a low sensitivity photodiodemay be optimal for sensor performance in high light conditions.Including a high sensitivity photodiode and a low sensitivity photodiodein a single pixel may therefore improve dynamic range. An imaging pixelof this type (with two discrete photosensitive areas) may sometimes bereferred to as a split pixel.

To increase high dynamic range in imaging pixels, imaging pixels mayalso be formed with an overflow capacitor. The photodiodes for theimaging pixel generate charge. After the charge exceeds a certain level,the excess charge may overflow from one of the photodiodes into astorage capacitor. The charge from both the storage capacitor and thephotodiodes may be read out after the integration time is complete. Thiseffectively increases the dynamic range of the pixel. The storagecapacitor in these types of pixels may be referred to as an overflowcapacitor.

FIG. 3 is a circuit diagram of an imaging pixel having a photosensitiveelement and a storage capacitor. As shown in FIG. 3, image pixel 34includes photosensitive elements 102-1 and 102-2 (e.g., photodiodes).Photosensitive element 102-1 (sometimes referred to as low-sensitivityphotodiode PD-L) has a first terminal that is coupled to ground. Thesecond terminal of photosensitive element 102-1 is coupled to transistor104 and transistor 106. Photosensitive element 102-2 (sometimes referredto as high-sensitivity photodiode PD-H) has a first terminal that iscoupled to ground. The second terminal of photosensitive element 102-2is coupled to transistor 106 and transistor 108. Transistor 110 may alsooptionally be coupled to photodiode 102-2.

The sensitivities of photodiodes 102-1 and 102-2 may be varied in anumber of ways. As an example, the doping concentrations of photodiodes102-1 and 102-2 may be modified to control the sensitivity,electrostatics, or other aspects of the photodiodes. In another example,the surface area (e.g., light collecting area) of photodiodes 102-1 and102-2 may be modified to control the sensitivity of the photodiodes.Different microlens arrangements may also be used to control thesensitivity of the photodiodes. Therefore, photodiodes 102-1 and 102-2may have different doping concentrations, light collecting areas, and/ormicrolens arrangements. Photosensitive areas 102-1 and 102-2 may becomposed of multiple photodiodes in parallel with parallel connectingtransistors 104, 106, 108, and 110. The sensitivity ratio of thehigh-sensitivity photodiode to the low-sensitivity photodiode may be atleast 3 to 2, at least 2 to 1, at least 3 to 1, at least 4 to 1, atleast 5 to 1, at least 10 to 1, less than 5 to 1, any intermediateratio, any larger ratio, etc.

Transistor 104 (sometimes referred to as threshold transistor 104) iscoupled between photodiode 102-1 and storage capacitor 116. Transistor106 is coupled between photodiodes 102-1 and 102-2. Transistor 108 iscoupled between photodiode 102-2 and floating diffusion (FD) region 112.Floating diffusion region 112 may be a doped semiconductor region (e.g.,a region in a silicon substrate that is doped by ion implantation,impurity diffusion, or other doping process). Gain select transistor 114has a first terminal coupled to floating diffusion region 112 and asecond terminal coupled to storage capacitor 116. Floating diffusion 112has an associated capacitance. Dual conversion gain capacitor 116 mayhave a first plate 116-1 (sometimes referred to as an upper plate or topplate) coupled to the second terminal of the gain select transistor.Dual conversion gain capacitor 116 may have a second plate 116-2(sometimes referred to as a lower plate or bottom plate) that is coupledto bias voltage supply terminal 118. Voltage supply 118 may provide avoltage V_(xx).

Source follower transistor 120 has a gate terminal coupled to floatingdiffusion region 112. Source follower transistor 120 also has a firstsource-drain terminal coupled to voltage supply 124. Voltage supply 124may provide a power supply voltage V_(AA). In this application, eachtransistor is illustrated as having three terminals: a source, a drain,and a gate. The source and drain terminals of each transistor may bechanged depending on how the transistors are biased and the type oftransistor used. For the sake of simplicity, the source and drainterminals are referred to herein as source-drain terminals or simplyterminals. A second source-drain terminal of source follower transistor120 is coupled to column output line 126 through row select transistor122. The source follower transistor, row select transistor, and columnoutput line may sometimes collectively be referred to as a readoutcircuit or as readout circuitry.

Reset transistor 128 may be coupled between gain transistor 114 andvoltage supply 130. Voltage supply 130 may provide a voltage V_(AA).Anti-blooming transistor 110 may optionally be coupled betweenphotodiode 102-2 and bias voltage supply terminal 132. Voltage supply132 may provide a voltage V_(AA). The bias voltage provided at biasvoltage supply terminals 118, 124, 130, and 132 may be the same or maybe different.

A gate terminal of transistor 104 (sometimes referred to as transfertransistor 104 or threshold transistor 104) receives control signal TXS.A gate terminal of transistor 106 (sometimes referred to as transfertransistor 106 or connecting transistor 106) receives control signalCONPD. A gate terminal of transistor 108 (sometimes referred to astransfer transistor 108) receives control signal TXL. A gate terminal oftransistor 128 (sometimes referred to as reset transistor 128) receivescontrol signal RST. A gate terminal of transistor 122 (sometimesreferred to as row select transistor 122) receives control signal RS. Agate terminal of transistor 110 (sometimes referred to as anti-bloomingtransistor 110) receives control signal AB. A gate terminal oftransistor 114 (sometimes referred to as gain transistor 114, conversiongain transistor 114, gain select transistor 114, conversion gain selecttransistor 114, etc.) receives control signal DCG. Control signals TXS,CONPD, TXL, RST, RS, AB, and DCG may be provided by row controlcircuitry (e.g., row control circuitry 40 in FIG. 2) over control paths(e.g., control paths 36 in FIG. 2).

Gain select transistor 114 and dual conversion gain capacitor 116 may beused by pixel 34 to implement a dual conversion gain mode. Inparticular, pixel 34 may be operable in a high conversion gain mode andin a low conversion gain mode. If gain select transistor 114 isdisabled, pixel 34 will be placed in a high conversion gain mode. Ifgain select transistor 114 is enabled, pixel 34 will be placed in a lowconversion gain mode. When gain select transistor 114 is turned on, thedual conversion gain capacitor 116 may be switched into use to providefloating diffusion region 112 with additional capacitance. This resultsin lower conversion gain for pixel 34. When gain select transistor 114is turned off, the additional loading of the capacitor is removed andthe pixel reverts to a relatively higher pixel conversion gainconfiguration.

To allow charge from photodiode 102-1 to overflow to storage capacitor116 and increase dynamic range of pixel 34, control signal TXS forthreshold transistor 104 may be set to an intermediate voltage levelduring the integration time of the pixel. When the charge levels inphotodiode 102-1 exceed a given level (set by control signal TXS),charge may overflow into capacitor 116. For example, the overflow chargemay follow path 134.

Including storage capacitor 116 in the imaging pixel increases thedynamic range of the pixel relative to embodiments where the storagecapacitor is not included. Additionally, including a low-sensitivityphotodiode 102-1 and a high-sensitivity photodiode 102-2 increases thedynamic range of the pixel relative to embodiments where the storagecapacitor is not included. Transistor 106 between photodiodes 102-1 and102-2 may increase the signal-to-noise ratio (SNR) or pixel 34 relativeto embodiments where the transistor is not included.

FIG. 4 is a timing diagram showing an illustrative method of operatingthe pixel of FIG. 3. The timing diagram shown in FIG. 4 may be performedfor each pixel during a respective frame. As shown, at t₁, controlsignals RST, DCG, TXS, TXL, and CONPD may be raised high to assert resettransistor 128, gain select transistor 114, transistor 104, transistor108, and transistor 106, respectively. Asserting these transistorscauses photodiode 102-1, photodiode 102-2, and floating diffusion region112 to be reset to reset voltage levels (e.g., a reset voltage providedby bias voltage supply terminal 130). Control signal AB (and,accordingly, transistor 110) is omitted from the timing diagram of FIG.4. However, it should be understood that if optional anti-bloomingtransistor 110 is included, control signal AB may be used to resetphotodiode 102-1, photodiode 102-2, and/or floating diffusion region112.

After photodiode 102-1, photodiode 102-2, and floating diffusion region112 are reset at t₁, control signals RST, TXL, and CONPD may be loweredat t₂ (e.g., transistors 128, 108, and 106 are deasserted). Controlsignal DCG remains high such that transistor 114 remains asserted.Control signal TXS is set to an intermediate voltage level at t₂. Whenthe charge levels in photodiode 102-1 exceed a given level (set bycontrol signal TXS), charge may overflow from photodiode 102-1 intocapacitor 116 (via path 134). TXS may remain at the intermediate levelthroughout the integration time, allowing overflow charge to flow fromphotodiode 102-1 to capacitor 116 throughout the integration time.

At t₃, the integration time may conclude and a readout time may begin.Control signal RS may be raised high to assert transistor 122 during thereadout period. The readout may begin with an E2 sample that is obtainedat t₃. The E2 readout may refer to readout of the overflow charge (thatis stored at capacitor 116). The E2 readout may include readout of asample level and a reset level for a double sampling.

In double sampling, a reset value and a signal value are obtained duringreadout. The reset value may then be subtracted from the signal valueduring subsequent processing to help correct for noise. The doublesampling may be correlated double sampling (in which the reset value issampled before the signal value) or uncorrelated double sampling (inwhich the reset value is sampled after the signal value is sampled,sometimes referred to as simply double sampling).

During the E2S sampling, conversion gain transistor 114 is asserted.This means that the E2S sample is a low conversion gain sample. Afterthe E2S sampling (e.g., obtaining the E2 sample level), the resetcontrol signal RST may be raised to assert reset transistor 128 at t₄and control signal TXS may be lowered to deassert transistor 104. Thisresets the voltage at floating diffusion region 112 to a reset voltage.Next, the E2 reset level (E2R) may be sampled at t₅. During the E2Rsampling, conversion gain transistor 114 is asserted. This means thatthe E2R sample is also a low conversion gain sample.

The E2R sample may be subtracted from the E2S sample to determine theamount of overflow charge at overflow capacitor 116. Because the samplelevel is obtained before the reset level, the E2 sampling is an exampleof uncorrelated double sampling (not correlated double sampling). Thereis therefore more noise than if correlated double sampling wasperformed. However, since the overflow charge is generated duringrelatively high light exposure conditions, the noise may notsignificantly impact the image data (e.g., the signal-to-noise ratiowill remain sufficiently high). Similarly, in FIG. 4 the E2 sampling isshown as being a low conversion gain sampling. The low conversion gainsampling has more noise than a high conversion gain sampling. However,again, since the overflow charge is generated during relatively highlight exposure conditions, signal-to-noise ratio will remainsufficiently high. If desired, correlated double sampling and/or highconversion gain readout may be used for the E2 sampling.

After the E2R sampling (e.g., obtaining the E2 reset level), the dualconversion gain control signal DCG may be lowered, deassertingtransistor 114 and placing the pixel in a relatively high conversiongain mode. The reset transistor may be asserted at t₆ to reset thefloating diffusion region 112. Then, the E1 reset level is sampled att₇. The E1 readout may refer to readout of the charge from thephotodiodes. The E1 readout may include readout of a reset level and asample level for a correlated double sampling.

After the E1R sampling (e.g., obtaining the E1 reset level), controlsignals TXL and CONPD may be raised to assert transistors 108 and 106.This results in charge from both photodiodes 102-1 and 102-2 beingtransferred to floating diffusion region 112. Then, the E1 sample level(E1S) may be obtained at t₉. The E1R sample may be subtracted from theE1S sample to determine the amount of charge present in the photodiodes102-1 and 102-2 at the end of the integration period. Because the samplelevel is obtained after the reset level, the E1 sampling is an exampleof correlated double sampling.

During the E1R and E1S samplings, conversion gain transistor 114 isdeasserted. This means that the E1R and E1S samples are high conversiongain samples. The E1 sample may be used for low light conditions.Therefore, the reduced noise associated with the high conversion gainduring the E1 sampling improves performance of the image sensor.

Including transistor 106 between photodiodes 102-1 and 102-2 such thatthe E1 readout includes charge from both photodiodes may improvesignal-to-noise ratio (SNR) relative to arrangements where the charge isread from photodiodes 102-1 and 102-2 independently. However, dependingupon the application of the particular image sensor, it should beunderstood that additional readout schemes may be used (e.g., readingoverflow charge from capacitor 116 and charge from photodiode 102-1simultaneously, independently reading charge from photodiode 102-1 and102-2, etc.).

FIG. 5 is a circuit diagram of an imaging pixel with a similararrangement to the imaging pixel of FIG. 3, but with the positions ofthe high-sensitivity and low-sensitivity photodiodes switched. Forsimplicity, duplicate descriptions for components already described inconnection with FIG. 3 will not be reproduced here. In FIG. 5,transistor 104 is interposed between low-sensitivity photodiode 102-1and floating diffusion region 112. Transistor 106 is still interposedbetween photodiode 102-1 and photodiode 102-2. Anti-blooming transistor110 is interposed between photodiode 102-2 and bias voltage power supplyterminal 132.

To allow overflow of charge from photodiode 102-1 to overflow to storagecapacitor 116 and increase dynamic range, control signal TXS forthreshold transistor 104 may be set to an intermediate voltage levelduring the integration time. When the charge levels in photodiode 102-1exceed a given level (set by control signal TXS), charge may overflowinto capacitor 116. For example, the overflow charge may follow path136. Path 136 passes through floating diffusion region 112, whereas path134 in FIG. 3 does not.

The arrangement of FIG. 5 may allow for an anti-blooming path forphotodiodes 102-1 and 102-2 while omitting a transistor compared to thearrangement of FIG. 3 (e.g., transistor 108 from FIG. 3 is omitted inFIG. 5). Depending on the technology node limitations, pixel pitch, andother design constraints of a particular image sensor, the arrangementof FIG. 3 or FIG. 5 may be advantageous.

FIG. 6 is a timing diagram showing an illustrative method of operatingthe pixel of FIG. 5. The timing diagram shown in FIG. 6 may be performedfor each pixel during a respective frame. As shown, at t₁, controlsignals RST, DCG, TXS, AB, and CONPD may be raised high to assert resettransistor 128, gain select transistor 114, transistor 104,anti-blooming transistor 110, and transistor 106, respectively.Asserting these transistors will cause photodiode 102-1, photodiode102-2, and floating diffusion region 112 to be reset to reset voltagelevels (e.g., a reset voltage provided by bias voltage supply terminal132).

After photodiode 102-1, photodiode 102-2, and floating diffusion region112 are reset at t₁, control signals RST, AB, and CONPD may be loweredat t₂ (e.g., transistors 128, 110, and 106 are deasserted). Controlsignal DCG remains high such that transistor 114 remains asserted.Control signal TXS is set to an intermediate voltage level at t₂. Whenthe charge levels in photodiode 102-1 exceed a given level (set bycontrol signal TXS), charge may overflow from photodiode 102-1 intocapacitor 116 (via path 136). TXS may remain at the intermediate levelthroughout the integration time, allowing overflow charge to flow fromphotodiode 102-1 to capacitor 116 throughout the integration time.

At t₃, the integration time may conclude and a readout time may begin.Control signal RS may be raised high to assert transistor 122 during thereadout period. The readout may begin with an E2 sample that is obtainedat t₃. The E2 readout may refer to readout of the overflow charge (thatis stored at capacitor 116). The E2 readout may include readout of asample level and a reset level for a double sampling.

After the E2S sampling (e.g., obtaining the E2 sample level), the resetcontrol signal RST may be raised to assert reset transistor 128 at t₄.Control signal TXS may also be lowered to deassert transistor 104. Thisresets the voltage at floating diffusion region 112 to a reset voltage.Next, the E2 reset level (E2R) may be sampled at t₅.

The E2R sample may be subtracted from the E2S sample to determine theamount of overflow charge at overflow capacitor 116. In FIG. 6 the E2sampling is shown as being an uncorrelated double sampling and lowconversion gain sampling. If desired, correlated double sampling and/orhigh conversion gain readout may be used for the E2 sampling.

After the E2R sampling (e.g., obtaining the E2 reset level), the dualconversion gain control signal DCG may be lowered, deassertingtransistor 114 and placing the pixel in a relatively high conversiongain mode. The reset transistor may be asserted at t₆ to reset thefloating diffusion region 112. Then, the E1 reset level is sampled att₇. The E1 readout may refer to readout of the charge from thephotodiodes. The E1 readout may include readout of a reset level and asample level for a correlated double sampling.

After the E1R sampling (e.g., obtaining the E1 reset level), controlsignals TXS and CONPD may be raised to assert transistors 104 and 106.This results in charge from both photodiodes 102-1 and 102-2 beingtransferred to floating diffusion region 112. Then, the E1 sample level(E1S) may be obtained at t₉. The E1R sample may be subtracted from theE1S sample to determine the amount of charge present in the photodiodes102-1 and 102-2 at the end of the integration period. Because the samplelevel is obtained after the reset level, the E1 sampling is an exampleof correlated double sampling. During the E1R and E1S samplings,conversion gain transistor 114 is deasserted. This means that the E1Rand E1S samples are high conversion gain samples.

FIG. 7 is a circuit diagram of an imaging pixel with a similararrangement to the imaging pixel of FIG. 3, but with overflow chargetransferred from a photodiode of a first row to a capacitor in asubsequent row. For simplicity, duplicate descriptions for componentsalready described in connection with FIG. 3 will not be reproduced here,and duplicate labels for components in row n+1 will not be provided forcomponents already labeled in row n.

In FIG. 7, each pixel 34 has the same arrangement as in FIG. 3. However,instead of transistor 104 being coupled between low-sensitivityphotodiode 102-1 and capacitor 116 of the same pixel (as in FIG. 3),transistor 104 of pixel 34-1 (in row n) is coupled between photodiode102-1 of pixel 34-1 and capacitor 116 of pixel 34-2 (in row n+1). Eachpixel 34 in FIG. 7 may optionally have anti-blooming transistor 110,similar to as discussed in connection with FIG. 3.

To allow charge from photodiode 102-1 to overflow to storage capacitor116 and increase dynamic range, control signal TXS for thresholdtransistor 104 may be set to an intermediate voltage level during theintegration time. When the charge levels in photodiode 102-1 exceed agiven level (set by control signal TXS), charge may overflow intocapacitor 116 of the subsequent row. For example, the overflow chargemay follow path 138.

Therefore, at the end of a given integration time, the overflow chargeassociated with row n will be stored in the storage capacitor 116 of rown+1. For faster readout, the E1 and E2 readouts may be performedsimultaneously. In other words, the overflow charge may be read from rown+1 using column line 126-2. The charge from photodiodes 102-1 and 102-2may be read from row n using a column line 126-1. Each column of pixelsmay therefore have two column lines instead of one, with each one of thetwo column lines being coupled to every other row of pixels. The firstof the two column lines may be coupled to all of the even rows in thearray whereas the second of the two column lines may be coupled to allof the odd rows in the array.

The arrangement of FIG. 7 may allow for faster frame rate, because theE1 and E2 sampling is performed simultaneously. Depending on thetechnology node limitations, pixel pitch, and other design constraintsof a particular image sensor, the arrangement of FIG. 3, FIG. 5, or FIG.7 may be advantageous.

FIG. 8 is a timing diagram showing an illustrative method of operatingthe pixel of FIG. 7. The timing diagram shown in FIG. 8 may be performedfor each pixel during a respective frame. Control signals fortransistors in row n are labeled (n) whereas control signals fortransistors in row n+1 are labeled (n+1).

As shown, at t₁, control signals RST(n), DCG(n), TXS(n), CONPD(n), RST(n+1), and DCG (n+1) may be raised high to assert reset transistor 128,gain select transistor 114, transistor 104, and transistor 106 in row nas well as transistors 128 and 114 in row n+1. Asserting thesetransistors will cause photodiode 102-1, photodiode 102-2, and floatingdiffusion region 112 in row n to be reset to reset voltage levels (e.g.,a reset voltage provided by bias voltage supply terminal 130).Additionally, floating diffusion region 112 in row n+1 will be reset toa reset voltage level. Although not shown in the example of FIG. 8, ifthe pixels include optional anti-blooming transistor 110, anti-bloomingcontrol signal AB may be used to assert transistor 110 and resetphotodiode 102-1, photodiode 102-2, and/or floating diffusion region112.

After the reset, control signals RST(n), CONPD(n), and RST (n+1) may belowered at t₂. Control signals DCG(n) and DCG(n+1) remain high such thattransistor 114 in rows n and n+1 remain asserted. Control signal TXS(n)is set to an intermediate voltage level at t₂. When the charge levels inphotodiode 102-1 exceed a given level (set by control signal TXS(n)),charge may overflow from photodiode 102-1 in row n into capacitor 116 inrow n+1 (via path 138). TXS(n) may remain at the intermediate levelthroughout the integration time, allowing overflow charge to flow fromphotodiode 102-1 in row n to capacitor 116 in row n+1 throughout theintegration time.

At t₃, the integration time may conclude and a readout time may begin.Control signal RS(n) and RS(n+1) may be raised high to assert transistor122 in rows n and n+1 during the readout period. At t₃, control signalRST(n) is raised to assert transistor 128 in row n. This resets floatingdiffusion 112 in row n. The reset voltage level for E1 (e.g., E1R) isthen sampled using column readout line 126-1. Simultaneously, the samplelevel for E2 (e.g., E2S) is sampled from capacitor 116 in row n+1 usingcolumn readout line 126-2.

At t₄, TXL(n) and CONPD(n) may be raised high to assert transistors 106and 108 in row n. This causes charge from photodiodes 102-1 and 102-2 tobe transferred to floating diffusion region 112 in row n for readout.Additionally at t₄, RST(n+1) is raised high to assert reset transistor128 in row n+1, thus resetting floating diffusion region 112 in row n+1.At t₅, the sample level for E1 (e.g., the charge from photodiodes 102-1and 102-2) is obtained using column output line 126-1. Simultaneously,the reset level for E2 is obtained using column output line 126-2. Thereset levels may be subtracted from the sample levels similar to asalready discussed above.

The readouts of FIG. 8 may ultimately have the same characteristics asin FIGS. 4 and 6. In other words, the E1 readout is a correlated doublesampling readout of charge from photodiodes 102-1 and 102-2 with a highconversion gain and the E2 readout is a double sampling readout ofoverflow charge with a low conversion gain. However, in FIG. 8 the E1and E2 readouts are performed simultaneously using two different columnlines, allowing for an increase in the frame rate.

An image sensor with pixels of the type discussed in FIGS. 7 and 8 mayhave an extra row of pixels at the bottom of the array of pixels toallow for readout from the last row of pixels using the same scheme asthe other rows of pixels. This extra row of pixels may sometimes bereferred to as a row of dummy pixels (because the dummy pixels are onlyused for readout and do not actually obtain image data). The dummypixels may have the same circuit arrangement as the other pixels in thearray. Alternatively, the dummy pixels may include only the componentsrequired for readout of the previous row's overflow charge (e.g., thephotodiodes may be omitted).

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the art. Theforegoing embodiments may be implemented individually or in anycombination.

What is claimed is:
 1. An imaging pixel comprising: a first photodiode;a second photodiode; a first transistor that is coupled between thefirst photodiode and the second photodiode; a floating diffusion region;a second transistor that is coupled between the floating diffusionregion and the second photodiode; a capacitor; a third transistor thatis coupled between the first photodiode and the capacitor; and a fourthtransistor that is coupled between the floating diffusion region and thecapacitor.
 2. The imaging pixel defined in claim 1, wherein the firstphotodiode has a different sensitivity than the second photodiode. 3.The imaging pixel defined in claim 1, wherein first photodiode has afirst sensitivity, the second photodiode has a second sensitivity, andthe second sensitivity is greater than the first sensitivity.
 4. Theimaging pixel defined in claim 3, wherein the second sensitivity is atleast two times greater than the first sensitivity.
 5. The imaging pixeldefined in claim 1, wherein the first photodiode is configured togenerate first charge in response to incident light, wherein the secondphotodiode is configured to generate second charge in response toincident light, and wherein a first subset of the first charge isconfigured to overflow from the first photodiode to the capacitorthrough the third transistor.
 6. The imaging pixel defined in claim 5,wherein a second subset of the first charge remains in the firstphotodiode until the end of an integration time and wherein, during areadout period, the first and second transistors are configured to beasserted to transfer the second subset of the first charge and thesecond charge to the floating diffusion region.
 7. The imaging pixeldefined in claim 5, further comprising: a readout circuit configured to:sample the first subset of the first charge; after sampling the firstsubset of the first charge, sample a first reset level associated withthe floating diffusion region; after sampling the first reset levelassociated with the floating diffusion region, sample a second resetlevel associated with the floating diffusion region; and after samplingthe second reset level associated with the floating diffusion region,sample a second subset of the first charge and the second charge.
 8. Theimaging pixel defined in claim 1, further comprising: a reset transistorthat is coupled between the capacitor and a bias voltage supplyterminal; a source follower transistor having a gate that is coupled tothe floating diffusion region; a column output line; and a row selecttransistor coupled between the column output line and the sourcefollower transistor.
 9. An imaging pixel comprising: a first photodiodehaving a first sensitivity; a second photodiode having a secondsensitivity that is greater than the first sensitivity; a firsttransistor that is coupled between the first photodiode and the secondphotodiode; a floating diffusion region; a second transistor that iscoupled between the floating diffusion region and the first photodiode;a capacitor; and a conversion gain transistor that is coupled betweenthe floating diffusion region and the capacitor.
 10. The imaging pixeldefined in claim 9, wherein the second sensitivity is at least two timesgreater than the first sensitivity.
 11. The imaging pixel defined inclaim 9, further comprising: a bias voltage supply terminal; and ananti-blooming transistor that is coupled between the second photodiodeand the bias voltage supply terminal.
 12. The imaging pixel defined inclaim 9, further comprising: a reset transistor that is coupled to thecapacitor and the conversion gain transistor.
 13. The imaging pixeldefined in claim 9, further comprising: a reset transistor that iscoupled between the capacitor and a bias voltage supply terminal; asource follower transistor having a gate that is coupled to the floatingdiffusion region; a column output line; and a row select transistorcoupled between the column output line and the source followertransistor.
 14. The imaging pixel defined in claim 9, wherein the firstphotodiode is configured to generate first charge in response toincident light, wherein the second photodiode is configured to generatesecond charge in response to incident light, and wherein a first subsetof the first charge is configured to overflow from the first photodiodeto the capacitor through the second transistor.
 15. The imaging pixeldefined in claim 14, wherein a second subset of the second chargeremains in the first photodiode until the end of an integration time andwherein, during a readout period, the first and second transistors areconfigured to be asserted to transfer the second subset of the firstcharge and the second charge to the floating diffusion region.
 16. Theimaging pixel defined in claim 14, further comprising: a readout circuitconfigured to: sample the first subset of the first charge; aftersampling the first subset of the first charge, sample a first resetlevel associated with the floating diffusion region; after sampling thefirst reset level associated with the floating diffusion region, samplea second reset level associated with the floating diffusion region; andafter sampling the second reset level associated with the floatingdiffusion region, sample a second subset of the first charge and thesecond charge.
 17. An image sensor comprising an array of imaging pixelsthat includes a first imaging pixel in a given row, the first imagingpixel comprising: a first photodiode; a second photodiode; a firsttransistor that is coupled between the first photodiode and the secondphotodiode; a floating diffusion region; a second transistor that iscoupled between the floating diffusion region and the second photodiode;and a third transistor that is coupled between the first photodiode anda capacitor of a second imaging pixel, wherein the second imaging pixelis in a row that is subsequent to the given row.
 18. The image sensordefined in claim 17, wherein the first imaging pixel comprises: anadditional capacitor; and a fourth transistor that is coupled betweenthe additional capacitor and the floating diffusion region.
 19. Theimage sensor defined in claim 18, wherein the additional capacitor iscoupled to an additional photodiode of a third imaging pixel by a fifthtransistor and wherein the third imaging pixel is in a row that isprevious to the given row.
 20. The image sensor defined in claim 17,wherein a first sensitivity of the first photodiode is lower than asecond sensitivity of the second photodiode.